A demand for high data rate and high-quality multimedia service has been raised in the radio communication field recently. In the mobile wireless environment, signals are usually deteriorated by fading or multipath delay phenomenon.
In such a communication channel, an influence of fading on amplitude of a signal might become serious or an influence of inter-symbol interference (ISI; Inter-Symbol Interference) might become serious by frequency selectivity of the channel, which lowers error performance and might disable communication depending on the case.
On the other hand, the OFDM technology is an effective method to reduce these influences of a multipath channel. That is because the ISI can be erased by inserting a guard interval longer than a delay spread of the channel.
Thus, the OFDM is employed in various next-generation wide-area WLAN (Wireless Local Area Network) of IEEE 802.11a, IEEE 802.11g, European HIPERLAN/2 and the like.
Ground digital audio broadcasting (DAB; Digital Audio Broadcasting) and digital video broadcasting are also proposed for the wide-area radio multiple access system. They are IEEE 802.16 wireless MAN standard and interactive DVB-T, for example.
Many of the OFDM systems use a fixed modulation scheme for all the carriers; this is for simplification.
However, there is a possibility that performance is improved by using a different demodulation scheme according to a channel state for each sub carrier of the OFDM system.
In this case, coherent or differential phase- or amplitude modulation scheme may be used. It includes BPSK, QPSK, 8PSK, 16QAM, 64QAM and the like, for example.
Each modulation scheme has a tradeoff between spectral efficiency and bit error rate (BER).
Thus, the best modulation scheme is such that the bit error rate is an allowable degree and the spectral efficiency can be maximized.
Such adaptive modulation schemes are disclosed in the documents mentioned below:
Non-Patent Literature 1: C. Ahn and I. Sasase, The effects of modulation combination, target BER, Doppler frequency, and adaptive interval on the performance of adaptive OFDM in broadband mobile channel, IEEE Trans. Consumer Electronics, vol. 48, no. 1, pp. 167-174, February, 2002
Non-Patent Literature 2: T. Nakanishi, S. Sampei and N. Morinaga, Variable coding rate OFDM transmission on one-cell reuse TDMA systems, IEICE Trans. Communications, vol. EB-88, no. 2, pp. 535-540, February, 2005
Non-Patent Literature 3: C. Ahn, S. Takahashi and H. Harada, Differential Modulated Pilot Symbol Assisted Adaptive OFDM for Reducing the MLI with Predicted FBI, IEICE Trans. Communications, vol. EB-88, no. 2, pp. 436-442, February, 2005
Non-Patent Literature 4: C. Ahn, S. Takahashi and H. Harada, Differential Modulated Pilot Symbol Assisted Adaptive OFDM for Reducint the MLI, Proc. of IEEE TENCON 2004, pp. 577-580, Chiang Mai, Thailand, November, 2004
As disclosed in the [Non-Patent Literature 1], in the Adaptive Modulation Scheme (AMS)/OFDM system, it is necessary to control a modulation level for each sub carrier at base station according to feedback information (FBI; Feedback Information).
The FBI includes evaluation results of channel state information (CSI) such as intensity and noise level of the respective sub carriers, for example.
It is general, here, to assume that accuracy of the FBI is indefinite and transmission of FBI can be ignored. However, in actual application, the transmission of FBI can be a serious problem.
Moreover, if an adaptive-modulated packet is to be transmitted from a base station to a mobile station after the base station controls the modulation level of each sub carrier, the mobile station needs modulation level information (MLI) for demodulation of the received packet.
Since the MLI is generally transmitted as data symbol, throughput of downlink of AMS/OFDM is deteriorated.
In the [Non-Patent Literature 2], such a scheme is proposed that a block of the AMS/OFDM sub carrier is fixed and an encoding rate is made variable for each block.
With this scheme, adjacent sub carriers are made into a block and assigned to the same modulation scheme among various encoding rates. By this arrangement, an amount of MLI transmission is reduced.
However, if the block size becomes large, the throughput is lowered by mismatch between the block modulation level and channel state.
Moreover, the number of required encoders and decoders is increased.
In the [Non-Patent Literature 3][Non-Patent Literature 4], a pilot-symbol-assisted adaptive OFDM system in differential modulation (DMPSA-AMS/OFDM) is proposed so that the MLI transmission amount is reduced.
In the DMPSA-AMS/OFDM system, the MLI is transmitted as a pilot symbol differentially modulated with FEC. Thus, the pilot symbol does not carry any information, and the transmission rate is not lowered.
However, delay time required for differentially demodulating and decoding the received pilot symbol so as to obtain MLI becomes longer.